What is a Social Network?

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This may seem, in this day and age, like an obvious question but sometimes the answer is harder than it seems. 

What is a social network is one of those questions that you think has a quick and easy answer until you try to articulate it.

So, using Facebook as an example here is a pretty good definition of a social network, nicely put together by Robin Al.  So, next time someone asks you, you have the answer at your fingertips!